Tuning screws are commonly used to tune high frequency circuits. To properly tuned high frequency circuits, the screw tip must provide a capacitive area which is small relative to the signal's wavelength. If this condition is met, the tuning screw helps to create a more uniform characteristic impedance over the signal path. Tuning screws are useful to compensate for conductors with an inductive section (i.e. a section lacking capacitance) due to missing material. An inductive section causes signal reflection and interrupts smooth signal flow. Reflections off the junction of conductors can be controlled by adding capacitance to the inductive section. Tuning screws with a tip diameter that is small relative to the wavelength of the signal help to eliminate reflections by creating a more uniform characteristic impedance over the signal path (see FIG. 1, which shows a prior art tuning screw inserted into an electrically conductive block as described above).
However, special challenges are encountered in using screws to tune circuits which operate at frequencies of greater than 50 GHz. The diameter of the screw must be small relative to the wavelength. At 50 GHz, a tuning screw diameter needs to be quite small. Tapping threads into a conductor with a diameter smaller than 1 millimeter becomes very expensive. Not only are the taps specially small, which can add to the cost, but they tend to break off during the tapping process, rendering the tap and the component useless. The parts cost rises as conductors with broken taps must be discarded or reworked. Moreover, the cost of making screws of small diameter and successfully inserting the screw into the conductor rises, again due to breaking of screws in the process and shearing off of screw heads during adjusting. Further, vibration and temperature cycling of the circuit tends to move the tuning screw from fine adjustment. This vibration induced screw movement has a serious impact on instrument performance.
What is needed is a tuning screw for microwave and millimeter wave components that provides a screw tip diameter that is small relative to the signal wavelength. What is further needed is a tuning screw that resists displacement from a tuned position, without external fixitives.